This apparatus is directed to a magnetostrictive force sensor and a magnetostrictive strain sensor in which a magnetic field in a closed loop magnetic circuit is generated, and at least one of the elements in the magnetic loop has magnetic properties responsive to changes in a load applied to that element.
Magnetostriction refers to stress or strain induced changes in magnetization, and has long been recognized. Bozorth determined that a tensile stress of 100 MPa (14.2 psi) increases the magnetic permeability of some materials by a factor of 100. Magnetostriction is measurable and may be applicable for sensing a number of different physical quantities. Physical properties measurable using magnetostriction include stress and strain in various mechanical configurations.
Conventional force sensors are modeled after electrical resistance type strain gages. In conventional force transducers, strain gages are installed on an elastic spring element subjected to the force to be measured. The spring element deflects under the force, and the magnitude of the strain changes. The strain gage is the sensing element, and it produces an electrical signal proportional to the measured force. Because strain gages are bonded to the spring element, the strains are transmitted directly to a very fine gage filament, causing a change in length and cross-sectional area, and thus, a change in resistance of the gage filament. Typically, the gage is incorporated into a Wheatstone bridge. Excitation circuitry energizes the bridge, and additional circuitry amplifies the electrical signal obtained from the bridge. The strain gage thus outputs a voltage proportional to the applied force.
Conventional force transducers of the strain gage configuration require precision machining of the spring element and proper installation of the gages. No reliable method exists to examine the bonding between the gage and the spring element, therefore, the bonding may deteriorate over time. Conventional force transducers also tend to be extremely delicate and require protection against mechanical damage. Furthermore, because the gages are bonded to the spring element, they possess a finite fatigue life and no overload protection. Such force transducers also require an excitation source and extensive signal conditioning because of the relatively low output voltages.